1. Field of the Invention
The present invention concerns a monitoring device for a gas-insulated cable. The present invention is more particularly concerned with a device for detecting and locating an internal arc and for detecting if a temperature threshold value is exceeded in a gas-insulated electrical cable.
2. Description of the Prior Art
An electrical cable of the above kind is described in U.S. Pat. No. 5,571,989.
It includes a sealed steel cylindrical outer case in which three phase conductors are disposed parallel to the axis of the case. An aluminum case is disposed between the conductors and the steel case, very near the latter, and constitutes a magnetic screen to reduce heating of the steel case. The cable is made up of a plurality of sections, a section being made up by butt welding on site cable elements assembled in the factory. Each cable element comprises a steel case element, an aluminum case element and three conductors held in position by insulative arms; a cable element of this kind is about ten meters long, a section comprising around ten elements being approximately 100 meters long. The cable is filled with insulative gas such as pressurized nitrogen. The above document also indicates that two adjacent sections are separated by a partition, one function of which is to provide a seal between the two sections.
A device for detecting an internal arc for a gas-insulated electrical cable of the above kind is described in French patent application 96 01 131.
The device for detecting an internal arc for a gas-insulated cable includes, disposed between the case and the screen, optical fibers equal in number to the number of cable sections and extending from one end to the other of the cable, each of the sections being monitored by a single fiber, the fiber assigned to monitoring a section including, in that section, a light sensor inserted in series in the fiber, the device including at each end, means for detecting the light conveyed by the optical fibers in the event of an internal arc in a section.
Another important parameter to be detected is the temperature of the sections. This temperature is an increasing function of I.sup.2 R losses in the section. The detection of a temperature above a predetermined threshold temperature can originate in abnormally high I.sup.2 R losses due to poor contact or to defective welds in the section concerned.
An aim of the present invention is to design a monitoring device using a single fiber to detect and locate an internal arc, even of low intensity, and for detecting if a threshold temperature is exceeded.